This invention is concerned with a rotary drilling technique for mitigating pressure-differential sticking of a drill string used in drilling a wellbore. This technique is particularly applicable for drilling deviated wellbores.
Pressure-differential sticking of drill pipe is discussed in a paper entitled "Pressure-differential Sticking of Drill Pipe and How It Can Be Avoided or Relieved" by W. E. Helmick and A. J. Longley, which was presented at the Spring Meeting of the Pacific Coast District, Division of Production, Los Angeles, California, in May 1957. In this paper it is said that the theory of pressure-differential sticking was first suggested when it was noted that spotting of oil would only free pipe that had stuck while remaining motionless opposite a permeable bed. This was particularly noticeable in a field where a depleted zone at 4300 feet with a pressure gradient of 0.035 psi per foot was penetrated by directional holes with mud having hydrostatic gradients of 0.52 psi per foot. It was concluded that the drill collars lay against the filter cake on the low side of the hole and the pressure differential acted against the area of the pipe in contact with the isolated cake with sufficient force that a direct pull could not effect release. This paper notes that the methods of effecting release of the pipe are spotting oil to wet the pipe, thereby relieving the differential pressure, or washing to water to lower the differential pressure, by reducing the hydrostatic head. Field application of the principles found in a study discussed in this paper show that the best cure for differential sticking is to prevent it by use of drill-collar stabilizers and, more important, conscientiously shortening the intervals of rest when pipe is opposite permeable formations.
The use of tubular drill string members formed to have grooves along continuous paths for reducing the area of its periphery engagement with the wellbore to thereby lessen the likelihood of the members becoming stuck due to differential pressure is described in U.S. Pat. No. 3,146,611 to Fred K. Fox.
In U.S. Pat. No. 3,306,378 there is described an invention which relates to drill collars used in a drill string for boring holes to maintain a stiff stem above the drill bit in order to counteract the tendency of the drill collars to flex and corkscrew and thus to increase the drilling weight without causing deviation of the bit. The invention is carried out by providing drill collars, having an eccentric hold therethrough, that are connected by means of tool joint connections on the ends thereof with drill pipe, whereby the drill collars gyrate in continuous contact with the wall of the borehole. Two or more collars are arranged symmetrically about the axis of rotation to maintain a uniformity of support on the wall of the borehole and provide the stiffness required to maintain linear alignment of the bit with the axis of rotation. In U.S. Pat. No. 3,382,938 there is described another method for controlling deviation of a drill bit from its intended course by providing drill collars which carry a series of spaced-apart pads extending radially from one side of the collar and having faces in wiping contact with the wall of the borehole. In U.S. Pat. No. 2,841,366 there is described a method and apparatus for drilling wells which are concerned with controlling and stabilizing the drill collars and bit at the lower end of a drill string. The action of the drill collars and bit is controlled and stabilized by the provision of an eccentric weight. At a point where the drill collars tend to buckle and bend there is provided a drill collar that has generally aligned upper and lower coupling portions and an eccentric intermediate portion. The eccentric intermediate portion will swing by action of centrifugal force in a circular path around the wellbore and have wiping engagement with the side of the bore, which tends to smooth the wall of the wellbore. As the eccentric portion revolves, the aligned portions are held concentric with the central axis of the wellbore and hold the drill bit vertically disposed such that the earth is penetrated in a manner to produce a straight, vertical bore. In U.S. Pat. No. 3,391,749 there is discussed a technique for preventing a well borehole from deviating from the vertical as it is being drilled by use of a drill collar which is eccentrically weighted with respect to its axis of rotation.
In U.S. Pat. No. 2,309,791 there is described a method and apparatus for cementing casing in a well wherein the casing is pushed away from the walls of the well, and any stringers of mud which tend to remain in place as cement slurry flows upward around the casing are broken up so that the casing can be completely surrounded by cement. The casing is provided with eccentric enlargements. Either by orientation of such enlargements with respect to the casing, rotation of the casing, or by a combination of the two, the casing tends to be centered in the hole. These eccentric enlargements can be carried by or comprised of a coupling, shoe, float collar, or any fitting placed in the casing string. Rotation of the eccentric enlargements disturbs the flow of an ascending cement column, tending to force it around all of the sides of the casing.
Wellbores and wells have been drilled to extend into the earth in directions other than vertical for various reasons. A need for such wells was early recognized and still exists today for tapping mineral reserves located beneath water bodies or located beneath other poorly accessible surface locations. For example, before the turn of the century, the Summerland Field located underwater near Santa Barbara, Cal., was drilled by whipstocking holes out under the water from land locations.
More recent developments have enabled ultrahigh angle wells to be drilled and completed. Techniques for drilling ultrahigh angles are sometimes referred to as "extended reach drilling", a term that has been coined to describe rotary drilling operations used to drill wellbores greater than 60.degree. from the vertical and wherein complex wellbore profiles may be used to extend the horizontal limits of wellbores. Such techniques may be used to provide a wellbore that extends from a surface location to a subsurface location spaced a great lateral distance therefrom.
Among the problems encountered in drilling deviated wells is that of differential sticking of drill pipe. This problem also is encountered in substantially vertical wellbores but the problem is much worse in deviated wellbores. In deviated wellbores the drill string tends to lie on the lower side of the wellbore and drill cuttings tend to settle and accumulate along the lower side of the wellbore about the drill string. This condition of having drill cuttings lying along the lower side of the wellbore about the drill string along with the usual filter cake on the wellbore wall presents conditions susceptible for differential sticking of the drill pipe when a porous formation is penetrated that has internal pressures less than the pressures existing in the borehole.
This invention is directed to alleviating the problem of differential sticking of the drill string by reducing the area of contact between the drill string and the wellbore wall and by sweeping the drill cuttings from the lower side of the wellbore into the main stream of the mud-return flow to better remove the cuttings from the wellbore.